One known gas sensor for detecting the concentration of a particular gas in exhaust gas is a gas sensor element including a cell having a pair of electrodes disposed on outer surfaces of a solid electrolyte body. Particularly, an example of a gas sensor for detecting the concentration of a particular gas in exhaust gas over the entire range is a stacked-type gas sensor element including two cells stacked with a measurement chamber formed therebetween (for example, Japanese Patent Application Laid-Open (kokai) No. 2013-185865). In this stacked-type gas sensor element, in order to adjust a voltage applied to one of the cells to be constant, the other cell pumps oxygen into/from the measurement chamber according to a current inputted from a control circuit. Each cell includes a pair of electrodes, and each electrode includes an electrode portion disposed on a forward end side of the gas sensor element, and a lead portion that electrically connects the electrode portion to an electrode pad disposed on a rear end side of the gas sensor element.
FIG. 7 is a cross-sectional view of the gas sensor element 10 described in Japanese Patent Application Laid-Open (kokai) No. 2013-185865. FIG. 7 shows a cross section of the gas sensor element 10 when it is viewed in a direction from its forward end to its rear end. FIG. 7 shows the arrangement of the lead portions in the cross section of the gas sensor element 10. The gas sensor element 10 includes a first cell 20, an insulating layer 30, a second cell 40, a protection layer 50, and a heater 60. The first cell 20 includes a solid electrolyte body 28 and first and second lead portions 24 and 26 electrically connected to a pair of electrode portions (not shown) disposed on the solid electrolyte body 28. The second cell 40 includes a solid electrolyte body 42 and third and fourth lead portions 44 and 46 electrically connected to a pair of electrode portions (not shown) disposed on the solid electrolyte body 42.